The sacrificial layers that are currently predominantly used for microfabrication processes such as micromachining are almost exclusively inorganic materials, the most commonly used being silica, i.e., silicon dioxide, SiO2. Aqueous hydrofluoric acid, HF, selectively etches SiO2 in the presence of silicon and silicon nitride, among other materials. This acid also etches phosphosilicate glass, PSG, faster than thermally grown SiO2, and can under-etch PSG over dimensions up to 2000 μm, with only minor damage to silicon or silicon nitride microstructures. HF also etches many other materials, including metal oxides and organic polymers. Although some non-silicon based materials, e.g., titanium and aluminum, can be used as sacrificial layers to be removed by an HF etch, the poor selectivity of this etch beyond oxides limits its usefulness with a wide range of relatively fragile microelectronic materials. Further, the toxicity of HF makes it inconvenient and/or hazardous for inexperienced users, and it poses difficult disposal requirements. HF-free etching solutions for aluminum are available, based on mixtures of acids and oxidants, e.g., concentrated phosphoric and nitric acids, hydrogen peroxide, and acetic acid, but are also incompatible with some fragile materials. Aside from oxide-based sacrificial materials, it has been shown that porous silicon can be employed as a sacrificial material in the fabrication of microsystems. Such can be removed by a final dissolution in an alkaline environment such as KOH.
Organic polymers—poly(imide), PMMA and photoresist—have also been used as sacrificial layers for micromachining. The removal of poly(imide) films by reactive ion etching (RIE) is compatible with most inorganic materials, but RIE has little selectivity in etching most organic materials. Sacrificial layers of photoresist can be removed by dissolution in acetone, or by thermal degradation, but these removal steps are incompatible with many other organic polymers. Photoresists that are used as sacrificial layers are also limited by their thermal sensitivity, that is, the photoresist film becomes insoluble in acetone after extended exposure to high temperatures. Sacrificial layers of photoresist are, therefore generally restricted to systems including only inorganic materials, and to processes having a minimal exposure to high temperatures. Polycarbonate, polystyrene, and polynorbornene have been reported as sacrificial materials for very specific applications, such as the preparation of sealed nanochannels. Poly(dimethylsiloxane), poly(methyl methacrylate) and epoxy-based polymers, removed by etching or thermal decomposition, have been used as sacrificial templates for the fabrication of metallic heat-exchangers.